1. Field of the Invention
The present invention relates generally to a CDMA (Code Division Multiple Access) mobile telecommunication system, and in particular, to an apparatus and method for reporting a voice service load to a mobile station in a system supporting both a voice service and a data service.
2. Description of the Related Art
In CDMA, a pilot signal used for initial acquisition and channel estimation is transmitted continuously or periodically in time. In an IS-2000 based system a base station transmits the pilot signal continuously, while in an HDR (High Data Rate) based system, the base station, transmits the pilot signal periodically.
In an IMT-2000 system, a mobile station, if it wants to receive a data service, measures the reception strength (Ec/Io) of a forward pilot signal transmitted continuously from the base station and reports the reception strength to the base station regardless of a handoff situation or a normal situation. The base station then transmits information about a data rate corresponding to the reception strength to the mobile station and provides the data service to the mobile station at the data rate on a supplemental channel (SCH).
Meanwhile, in the HDR system proposed for a high data rate service, the mobile station, if it wants to receive a data service, measures the reception strength (C/I) of a forward burst pilot signal, selects a data rate and a sector corresponding to the reception strength regardless of a handoff situation or a normal situation, and transmits the data rate and the sector as a signal to the base station on a reverse DRC (Data Rate Control) channel in each slot. Upon receipt of forward data rate requests from mobile stations within the sector, the base station schedules user data according to the amount of user packet data and the requested data rates, determines a mobile station to be serviced in next slots after the current packet is completely transmitted, and provides the data service to the selected mobile station at the requested data rate. Here, the base station transmits a traffic channel to a mobile station in time division by scheduling.
The base station can transmit a pilot signal in the above two methods and the mobile station implements a handoff according to the pilot signal. For handoff, the mobile station manages neighboring base stations. The base stations are grouped into sets. The base station sets are categorized as an active set, a candidate set, a neighbor set, and a remaining set. If two or more base stations belong to the active set, the mobile station is placed in a handoff situation. If only one base station exists in the active set, the mobile station is in a normal situation. Base station sets and set management associated with the present invention will be described below.
For voice service, the mobile station usually performs a soft handoff in which it communicates with all the base stations in the active set. For data service, the mobile station performs the soft handoff or a hard handoff in which it selects one of the base stations in the active set and communicates with the selected base station in a handoff area. To determine which base stations belong to the active set, the mobile station measures the reception power of pilot signals received from the base stations and reports the measurements to the network. If the reception power measurement is at a threshold level or above, the network requests that the mobile station includes in the active set the base station whose reception power is at or above the threshold level. The mobile station then classifies that base station in the active set as requested.
In the case of a hard handoff for data service, the mobile station selects a base station corresponding to the strongest of the reception power of pilot signals from the base stations in the active set and reports the selected base station to the network. Communication with the base station corresponding to the strongest pilot reception power is favorable for voice service because as the base station offers stronger pilot reception power, it can provide a better quality voice service. In data service, however, the quality of a data service and a data rate available to the mobile station are determined according to the transmission power of the base station. Therefore, the pilot reception power cannot be the only criterion by which the mobile station selects a base station for handoff in order to receive a good quality data service.
FIG. 1 is a flowchart illustrating a signal reception procedure in a mobile station to select a base station in a handoff situation or a normal situation in a conventional system employing a continuous pilot transmission scheme.
Referring to FIG. 1, the mobile station measures the reception strengths (Ec/Io) of pilot signals from all base stations in an active set that the mobile station manages for handoff in step 101. In step 103, the mobile station reports information about the strongest reception power and a base station corresponding to the strongest reception power to the network. The network determines a data rate available to the mobile station based on the reported information and transmits the determined data rate as a signal to the mobile station. The mobile station receives the determined data rate from the network in step 105.
FIG. 2 is a flowchart illustrating a signal reception procedure in a mobile station to select a base station in a handoff situation or a normal situation in another conventional system employing a periodic pilot transmission scheme.
Referring to FIG. 2, the mobile station measures the reception strengths (C/I) of pilot signals from all base stations in an active set that the mobile station manages for handoff in step 201. If two or more base stations belong to the active set, the mobile station is placed in a handoff situation. If only one base station exists in the active set, the mobile station is in a normal situation. In step 203, the mobile station determines a base station and a data rate corresponding to the strongest reception power. The mobile station transmits information about the determined base station and data rate to the network in a DRC symbol that is transmitted in every slot in step 205.
As described above, the mobile station selects a base station to provide a data service and a data rate for the data service based on pilot reception power without considering transmission power that the base station can spare for the data service in the conventional systems. The pilot signal is a signal transmitted with fixed power from a base station. When a data service and a voice service are provided at the same time, the base station first determines transmission power for the voice service (or voice load) and then assigns the remaining power to the data service. In other words, even though the reception power of a pilot signal is great, it does not imply that the reception power of the data service is great. Hence, it is preferable that the mobile station selects a base station with the highest data service power for data service. Therefore, the best base station and an optimal data rate cannot be determined based on pilot reception power alone.